This book has been on my to-read list since around 2012. Finally, 14 years later I pick it up and give it a try. And I'm glad I did.
The very early letters between Ginsberg and Cassady in the 1940s are not so interesting. Neither of them has yet to find his 'voice.'
However, Cassady's letters in the early 50s are superb and Ginsberg's letters from his travels in Mexico around 1954 are beautifully and descriptively written.
As some other reviewers have pointed out, there are some saucy letters between Ginsberg and the bi-curious Cassady here which were left out of the so-called Collected Letters compiled by Carolyn Cassady. Someone really needs to put out a complete 'Collected Letters' of Cassady which includes the full Joan Anderson Letter (since it has now been discovered), these letters to Ginsberg and the letters from prison (collected in Grace Beats Karma). Or, alternatively, if you have this volume, the Collected, Grace Beats Karma, and the separately published Joan Anderson Letter (the one that inspired Kerouac to develop his spontaneous prose method), then you have pretty much most of them.
It's a real pity Neal didn't work harder on his writing because if his letters are any indication to go by, the man had talent.